The present invention concerns lightweight, electrochromic (EC) mirrors for vehicles.
There is a constant need for weight reduction in vehicles in order for vehicle manufacturers to stay competitive. The reasons are many and varied, but include such things as meeting customer demands for improved gas mileage through reduced vehicle weight; obtaining the benefit of government incentives through improved gas mileage, reduced pollution, and reduced curb weight; reducing part cost by reducing material costs; and other engineering and marketing considerations. Concurrently, there are reasons to improve mirrors in ways which may increase weight, such as to increase the size of mirrors to increase the reflected viewing area; to increase the structural strength of mirror elements to reduce distortion of reflected images; and to increase the structural support provided for mirrors to withstand vibration and pendulum-like harmonic movement of the mirrors when a vehicle is moving along a highway. Clearly, there is tension between the need to reduce vehicle weight and the need to improve mirrors.
In addition to the above, vehicle mirrors have environmental problems unique to their application. Automotive mirrors must be stable under high vibration conditions and also must remain undistorted in a wide range of temperatures and operating conditions in order to be effective. Due to inherent limitations in vehicle design and mirror support design, most interior and exterior rearview mirrors are supported in cantilever. However, the dynamic vibrational stability of a mirror is inversely proportional to its cantilevered mass and, accordingly, vibration is often a problem in vehicle mirrors, particularly in heavier mirrors. Therefore, lighter mirror weights and those having shorter cantilever arms are desired. Improvements have been made in mirror housings to make them lighter, stiffer, and more aerodynamic. However, glass remains a large percentage of the overall weight of mirrors. The reason glass continues to be used is because it has high transmission of light and it is very durable. Unfortunately, glass has a relatively high specific gravity. Further, glass is somewhat flexible and deformable, unless the thickness of glass mirror elements is kept at or above the 2.2 mm thickness dimension that most conventional mirrors use. In other words, prior to the present inventive improvements, the thickness of glass must be maintained at thickness levels sufficient to provide adequate stiffness in order for the glass to remain rigidly flat enough to support a reflector to provide a distortion free reflection.
Vehicle manufacturers have recently begun manufacturing vehicles with interior and exterior EC mirrors due to their advantages over non-EC mirrors. EC mirrors have the advantage of undergoing controlled dimming to eliminate glare from bright lights, such as to eliminate glare in a driver's eyes from the headlights of other vehicles. However, EC mirrors tend to be heavy because they utilize two glass elements with an EC material (solution phase, solid phase, gel phase, or a hybrid thereof) therebetween. The reflectance of the mirror is controlled by creating an electrical voltage potential across the EC layer as glaring lights are sensed behind the vehicle. Unfortunately, EC mirrors tend to weigh more than conventional mirrors due to the "double thickness" of glass.
There are significant mounting problems associated with using "extra" thin glass elements (i.e., glass elements having a thickness of about 1.6 mm or less), because these extra thin glass elements are so thin that they are susceptible to deformation and will unacceptably flex in response to relatively low stress. Non-uniform deformation of the glass elements in a direction perpendicular to the surface of the glass elements of as little as a tenth of a millimeter over a small area (i.e., relatively sharp or sudden deformation) can result in noticeable, measurable, and objectionable distortion of reflections. The noticeability of these deformations will depend upon the sharpness of the deformation, its length and location on the mirror, and numerous other factors, making it difficult to quantify these deformations. However, this does not detract from their significance and, in fact, it adds to the difficulty in dealing with them. Notably, these unacceptable deformations can occur at any time in the EC mirror manufacturing process, and can even occur well after the mirror manufacturing process, such as during assembly of the mirror into a mirror housing or onto a vehicle or during normal "post-assembly" relaxation of components. Deformations can also occur when thermoplastics creep in reaction to relaxation of internal and external stresses and/or when distortion occurs while in service (e.g., due to thermal and other environmental stressors). The problem is compounded in thin glass elements separated by a solution phase EC material, a gel phase EC material, or hybrid thereof, because the front and rear elements do not reinforce each other to provide a rigid "compound" beam strength like they would if they were fixedly secured together.
By way of example, in many exterior EC vehicle mirrors, a double-glass-element EC mirror is adhered to a heater pad, which is in turn adhered to a "flat" support surface of a carrier. In exterior mirrors, a power pack adjustment mechanism for motorized adjustment of the angle of the reflector is attached to the carrier (such as by snap-attachment or other quick-attach mechanical means). The assembly (whether or not it includes a power pack adjustment mechanism) is then operably mounted in a mirror housing, which mirror housing is adapted for attachment to the vehicle.
Problematic deformation to a mirror having flex-sensitive thin glass elements can occur, for example, as a result of imperfections in the carrier support surface. These imperfections telegraph through the heater pad and through the adhesive to the rear glass element. Since the flex-sensitive thin glass elements are not particularly stiff, it is very difficult to assemble the thin glass element onto a supporting assembly without unacceptably deforming it. If the carrier is not exceptionally flat and defect free, then the thin glass elements will likely be distorted optically upon being bonded to the carrier. If the carrier is very weak, then the adhesive itself may deform the rear glass element (i.e., the element having the reflector disposed thereon) as the adhesive cures. Even when the carrier is flat, the adhesive can distort and stress the rear glass element as it is positioned on the carrier, such as by entrapping bubbles of air and/or by stressing and distorting the rear glass element during the attachment/bonding/curing process for the adhesive. Another problem occurs when the weak/flat carrier deforms upon attachment to its support structure, such as when a carrier is attached to a power pack actuator. For example, many power pack adjustment mechanisms are snapped to the carrier, with attachment fingers on the carrier resiliently flexing to receive and grip the power pack. However, the attachment fingers extend from a back surface of the carrier. Flexing the attachment fingers results in torsionally bending and distorting a small area of the support surface on the carrier. This results in a slight ripple or surface change in the support surface. These "slight ripples" read through the heater pad to the rear glass element with little or no smoothing or "dampening" by the heater pad, thus causing distortion in the element having the reflector thereon.
Another problem is that attaching the EC mirror subassembly to its housing and/or attaching the mirror housing to the vehicle can cause significant distortions and considerable transfer of stresses to the mirror elements. Yet another problem is that known processes and designs for supporting mirror glass elements are inherently variable and simply are not tolerant of the myriad variances which occur in processing and assembling all of the components. Imperfections in carriers can come from limitations of and normal process variations caused by standard molding techniques and/or part design. They may include as-molded part problems (e.g., sinks and non-uniformities in the flat support surface resulting from poor part or mold design or that result from poor cooling of the part prior to ejection from the mold) and after-molding part problems (e.g., scratches, creep, and dimensional change to the support surface due to plastic relaxation and/or distortion subsequent to molding of a part).
It was noted above that imperfections in a carrier telegraph through or "read through" the heater pad onto flex-sensitive glass elements. Part of the problem is that the known EC mirrors use an aggressive adhesive to assure secure bonding of their rear glass elements to the carrier. However, the aggressive adhesive results in the rear glass elements being held so tightly to the surface of the carrier, that any defect, depression, or imperfection on the surface reads through to the thin glass element. Further, the heater pad incorporates a closed-cell foam that is resistant to absorbing water. However, the closed-cell foam does not adjust or "flow" to smooth out the effect of protrusions that extend into one side of the foam. Instead, protrusions that impact one side of the foam read through to the opposite side of the foam. This is believed to be due to the fact that the closed-cell "bubbles" in foam sheets now used in mirrors and heater pads tend to move in a direction perpendicular to the sheet of foam rather than to compress, deform, or shift parallel to the foam sheet. Thus, defects on the support surface tend to read through to the glass element causing unacceptable surface distortion in the glass element, unless the glass element is thick enough to resist such distortion.
Another problem comes from thermal-related circumstances. Mirror assemblies undergo thermal expansion and contraction due to environmental temperature changes, both in terms of actual temperature and in terms of the rate of change in temperature. Glass expands/contracts a different amount and at a different rate than other components (e.g., plastic and metal components) of known mirrors, which results in transient, semi-permanent and permanent distortions, non-uniformities, and other problems both during the manufacturing process and also during assembly onto a vehicle and when the vehicle is in service.
Despite the problems with using flex-sensitive thin glass elements in vehicle mirrors, there are many advantages that could result from using thinner glass elements in EC mirrors. EC mirrors include a front glass element that is intended to be substantially transparent and a rear glass element having a reflector on either its front or rear surface. However, the front glass element reflects approximately 4 to 7 percent of the light impinging on it. As a result, EC mirrors will reflect a double image when viewed from an angle, with the double image including a primary image generated by light reflecting off of the reflector and a light ghost image reflected off of a front surface of the front glass element. (For example, see FIGS. 24 and 25.) The double image is only detectable by a human eye, when the distance between the primary image and the reflected image is great enough for the human eye to distinguish two separate images. This distance between the primary image and the ghost image is directly proportional to the thickness of the glass elements (and also of the angle of viewing). Therefore, a very thin front glass element results in a much shorter distance between the surfaces reflecting the primary image and the ghost image. Thus, having a thin front glass element is a very desirable feature, since the tendency to reflect visible ghost images (i.e., the tendency to reflect visible double images) is greatly reduced. Even apart from weight reduction, if the reflector is on the fourth surface, having thin front and rear glass elements greatly reduces these double images.
Accordingly, a mirror having reduced weight and that solves the aforementioned problems, as well as provide the aforementioned advantages, is desired.